This invention relates to endoluminal prostheses such as vascular repair devices, and in particular intravascular stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel or coronary artery, to maintain the lumen""s patency. Stents are particularly useful in the treatment of atherosclerotic stenosis and are most frequently used in connection with coronary angioplasty.
Stents are tubular, usually cylindrical devices which hold open a segment of blood vessel or other body lumen. They also are suitable to support and hold back a dissected arterial lining that can occlude the lumen. At present, numerous models of stents art marketed throughout the world. While some of these stents are flexible and have the appropriate strength and rigidity needed to hold open a lumen such as a coronary artery, each stent design typically represents a compromise between the stent""s flexibility and its radial strength. What has been needed, and heretofore unavailable, is a stent which has a high degree of flexibility so that it can be advanced through tortuous lumen and readily expanded, and yet have the mechanical strength to hold open the lumen or artery into which it is implanted and provide enough stiffness to prevent catheter rotation. Also needed is a stent that provides adequate vessel wall coverage, permits a large expansion range and offers more conformability.
At least some in the stent industry also perceive a problem with xe2x80x9cfishscaling.xe2x80x9d Fishscaling, describes the twisting or bending of stent struts, which results in the struts not conforming to a generally cylindrical plane around the circumference of the stent. Fishscaling can result from the manufacturing process, and also can occur during the stent placement process, such as when portions of the stent surface are forced outward as the stent bends while advancing through tortuous lumen. Some in the stent art believe that fishscaling can damage the blood vessel through which the stent is being advanced. Therefore, there is a perceived need for a stent that reduces or eliminates fishscaling.
The present invention is directed to an endoluminal prosthesis, such as an intravascular stent, which is highly flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but which is strong and stable enough radially in its expanded condition to maintain the patency of a body lumen when the stent is implanted therein. The stent also reduces fishscaling, provides more uniform connectivity, and decreases unsupported surface area.
The stent of the present invention includes a plurality of generally cylindrical elements, also known as rings, that are interconnected to form the stent. The stent typically is mounted on a balloon catheter if it is balloon expandable, or it can be mounted on a catheter without a balloon if it is self-expanding.
Each of the cylindrical rings or elements has a proximal end and a distal end and a cylindrical plane defined by a cylindrical outer wall surface that extends circumferentially between the proximal end and the distal end of the cylindrical ring. The rings are aligned along a longitudinal axis, with each having numerous peaks and valleys. These peaks and valleys are defined by adjacent bar arms, including, generally linear and nonlinear bar arms, with the nonlinear bar arms having an undulating shape. The linear bar arms have a first arm axis and the nonlinear bar arms have a second arm axis, the first arm axis is parallel to the longitudinal axis of the stent, and the first and second arm axes are at acute angles to each other. At least one link is used to connect each cylindrical ring to an adjacent ring to form the stent. The links are connected to a central portion of the nonlinear bar arm on one ring and a peak of the adjacent ring.
An embodiment of the invention has alternating rows of at least one link consisting of nonlinear links, while the other rows of at least one link consist of linear links. This design enhances flexibility with the use of the nonlinear links, but also provides stiffness which prevents catheter rotation with the use of the linear links. In this embodiment, the nonlinear links are offset by ninety degrees from the adjacent linear links.
In one embodiment, the peak that is connected to the nonlinear link is defined by a v-shaped bar arm. Alternating cylindrical rings have at least one v-shaped bar arm which forms at least one peak of the ring. In each alternating ring, the v-shaped bar arm is connected to the adjacent nonlinear bar arm on one side, and the adjacent linear bar arm on the other side.
The cylindrical rings define certain patterns, one pattern called a unit, involves one nonlinear bar arm disposed in-between two linear bar arms. In this embodiment, the units on adjacent cylindrical rings are mirror-images of one another. This design with the mirrored units provides more uniform connectivity. Another pattern found in the cylindrical rings, is a ring portion shaped like a figure-eight. The figure-eight portions are defined by a linear bar arm and portions of two non-linear bar arms. Another aspect of the stent design is that the peaks and valleys of one cylindrical ring are separately disposed in-phase with the peaks and valleys of the adjacent cylindrical ring.
Typically, a balloon expandable stent is made from a stainless steel alloy or similar material. The cylindrical rings of the stent are plastically deformed when expanded by the balloon. The cylindrical rings of the stent can expand radially outwardly without a balloon when the stent is formed from a superelastic alloy, such as nickel titanium (NiTi) alloys. These so-called xe2x80x9cself-expandingxe2x80x9d stents expand upon application of a temperature change or when a stress is relieved, as in the case of a pseudo-elastic phase change.
The number of peaks, valleys, links, and cylindrical rings can be varied as the application requires. When using nonlinear or flexible links, the link typically does not expand when the cylindrical rings of the stent expand radially outwardly, but the links do continue to provide flexibility and to also provide a scaffolding function to assist in holding open the artery. Further, because the links do not expand or stretch when the stent is radially expanded, the overall length of the stent is substantially the same in the unexpanded and expanded configurations. In other words, the stent will not appreciably shorten upon expansion.
The stent can be formed from a tube by laser cutting the pattern of cylindrical rings and flexible links in the tube. The stent also may be formed by laser cutting a flat metal sheet in the pattern of the cylindrical rings and links, and then rolling the pattern into the shape of the tubular stent and providing a longitudinal weld to form the stent